I had the same Problem -- blank Screen when trying to upgrade from 5.0.14 to 6.0.0a on a QNAP TS 259 Pro+.

It worked after setting

eeaccelerator.enable = 0

in the php.ini.

And even after successfull upgrade, OC 6.0.0a works only with eeaccelerator = 0, setting it back to 1 leads to a "page not found" or to a blank screen. So it very much seems like an OC bug handling the php accelerator.